Jim Michalak's Boat Designs

(28FEB98) This issue hopes to show the basics and effects of water
ballast on static stability. Next issue, about 15 March, I'll continue
the subject with a look at putting water ballast on a real boat.

MORE NET TREASURE...

If you click on Blue
Peter Marine.you will take a trip to Australia. This particular
page has a free"Hullform6S" program you can download. This is a
"thinking" program that seems very good at calculating displacement and
stability and lots of other things that can be very difficult to do by
hand. It is not really a CAD program to allow you to easily draw a
picture of a boat. The best approach seems to be to design the boat
first, and reconstruct it in the Hullforms program by typing hull
offsets in the proper place. The program runs very nicely in DOS even
on old slow machines.

Left:

KARL JAMES' JEWELBOX

Contents:

Contact info:

WATER BALLAST

WATER BALLAST....

I spent a few hours (days) thinking about water ballast and how it
works. In general I was bothered by the comment you will often see that
only the portion of the water ballast that is actally raised above the
waterline by the heeling of the boat is effective in trying to right
the boat.

From my study I concluded that the above statement seems true for
external water ballast, but not for internal water ballast. In the
study I looked at five different ballast configurations on a very
simple "boat" model. Each configuration was run through the Hullform6S
program and the righting moment curves from 0 to 90 degrees of heel
were determined and compared.

...RIGHTING MOMENT...

Figure 1 shows a diagram of how to figure the righting moment of a boat
at a certain angle of heel. This is a static analysis which is to say
the boat is not accelerating. It can be moving, but all the forces on
the boat are in balance. If the boat is being pitched and rolled about
in angry seas, a much more complex analysis is required. Also, the
distribution of the ballast (as opposed to simply the location of its
center of gravity) becomes a factor in a dynamic analysis.

What we have here is the wind's pressure on the sail, up high, and a
balancing load on the keel or fin, conspiring to tip the boat over.
That "moment" or torque, is counteracted by the weight of the boat
pushing down and the buoyancy of the boat pushing up. These last two
forces are not in line with each other when the boat heels but are
separated by a distance called the "righting arm". If the weight of the
boat in is "pounds" and the length of the righting arm is "feet", the
righting moment is measured in "foot pounds".

To figure the foot pounds of the righting moment, you need to know the
weight of the boat and the location of the center of that weight - the
"CG". Also you need to know the location of buoyancy of the heeled
boat. (The buoyancy itself is the same as the weight in a static
analysis.)

...FIGURING CG...

The CG is sort of the "average" location of all the weights. To get
stated figuring a CG location, you must have a reference line and in
these examples I will use the bottom of the boat as the reference.
We're only going to figure the vertical (up and down) CG in these
simple examples, but in a complex project you might also figure the
location laterally (side to side} and longitudinal (along the length)
locations.

Let's say our boat only had two elements, the hull and all its contents
weighing 500 pounds with that weight centered 2' above the bottom, and
let's say 250 pounds of internal ballast centered 3" (which is .25')
above the bottom. So the total weight is 750 pounds.

To find the CG, you multiply each weight by its vertical location, add
all those pieces, and then divide that sum by the total weight. So the
example calculation would be CG = (500x2)+(250x.25) all divided by
(500+250 and that equals (1000+62.5) / 750 = 1.42' above the base line.

So the effect of the 250 pounds of ballast was to lower the CG from 2'
to 1.42', a difference of 7".

In this calculation the makeup of the ballast is not a factor. The only
factors are the weight and location of the ballast. The only way the
ballast material could be a factor is if it were of such density that
it could be centered closer to the bottom. For example if the ballast
were water inside a rectangular tank 6" deep, 4' wide and 2' long, it
would amount to 4 cubic feet of water which is about 250 pounds and it
would center at 3" above the bottom and provide the above CG location
of 1.42". If we switched to lead which is 11 times denser than water,
we might have a plate only .54" thick. We could mount it centered .27"
(which is .0225') off the bottom instead of 3". The new CG would be
((500x2)+(250x.0225))/750=1.34'. So the CG of this lead ballasted boat
is lower by less than an inch.

...FIGURING BUOYANCY LOCATION...

This is very hard to do by hand. I'm going to use Hullform6S as a tool
to do this. To keep lots of variable from getting in the way, I'm going
to use the above pictured "boat" as the example. It is just a box, 16'
long, 4' wide, 2'deep. It weighs 500 pounds with its unballasted weight
centered on the top of the box, 2' above the bottom. I'm going to
ballast the box in four different ways, roll each example over at 10
degree intervals to 90 degrees, and record and plot the righting
moments as predicted by Hullform6S.

...EXAMPLE A...

So example A will be the above unballasted 500 pound box.

...EXAMPLE B...

Example B is shown below. It's the same as A exept it has a 250 pound
lead fin with the weight of the fin centered 2' below the bottom. So
the CG of this combination is .66' above the boat's bottom. But, the
250 pounds of lead displaces some water, right? Its volume amounts to
.36 cubic feet of lead displacing the same amount of water which is 22
pounds of water trying to float the lead back up. So the total ballast
effect of the lead while it is under water is actually about 228
pounds. If the boat heels to the point where the lead is totally out of
the water, it has 250 pounds of ballast effect.

...EXAMPLE C...

Example C has the same 250 pound fin configuration as example B except
a water filled tank, 2' long, 6" wide, and 4' deep, is used instead
oflead. That amounts to 250 pounds of ballast water centered 2' below
the hull so the CG of the total boat/ballast combination is the same
.66'above the bottom. Again, the 250 pounds of external ballast
displaces some water. The water ballast displaces its own weight in
water! So the ballast weight is exactly balanced by the buoyancy of the
displaced water. So this underwater tank has no ballast effect as long
as it is under water. When the boat heels enough to raise it out of the
water, it becomes effective.

...EXAMPLE D...

Example D is really the same as C except the ballast tank has been
moved inside the hull. So now it is 6" deep, 4' wide and 2' long. The
ballast weight is centered 3" above the bottom of the hull and the
overall CG is at 1.42' above the bottom. We don't have a separate
external tank displacing water. But compared to example A the hull
sinks a bit deeper to float that extra weight so the statics of the
Hullforms analysis is a bit different.

...EXAMPLE E...

Example E has a 250 pound V shaped ballast tank on its bottom. Whether
it might be called internal ballast or external ballast is in the eye
of the beholder. I included it because I thought it might represent
some water ballasted trailer sailers you can buy.

The results of the Hullform6S study are shown below:

Curve A is, I think, pretty typical of a light flat boat with no
ballast. The maximum righting moment of about 450 foot pounds is
reached quite quickly although I think a real boat would have the peak
at about 20 degrees. The boat looks to capsize at about 45 degrees
heel. My experiences with Jinni, about this size and weight, were
similar. It capsized twice in the time I had it. Both time it went over
well before it shipped any water over the rail.

Curve B shows the how effective metal outside ballast can be. Not only
is the maximum righting moment about twice that of example A, it still
has substantial righting ability at 90 degrees of roll. It will tend to
roll upright at any angle of heel up to about 110 degrees.

Curve C, external water ballasted fin, is an interesting one. Until the
fin starts to exit the water as the boat rolls, it has no effect. When
fully out of the water (about 80 degrees of roll) it is as effective as
metal. In between its righting moment goes to about zero. If rolled to
about 50 degrees it would stay there until acted upon by an outside
force such as a wave or maybe the crew moving about. If rolled to a
greater heel angle it will try to return to 50 degrees. If rolled to a
lesser degree it will continue to roll fully upright.

Curve D, internal ballast, has about 50% greater maximum righting power
than no ballast or the external water fin. It should easily outsail a
water fin boat up until it capsizes at about 65 degrees of heel. At
that point the water fin boat gets back on its feet while the internal
ballast boat flops over.

Curve E, V ballast tank, cuts across about everything as a compromise.
It doesn't have the initial stability of the internal ballast boat, but
it has positive stability at high roll angles. I've heard water
ballasted production boats behave this way.

...CONCLUSIONS...

I can't see any obvious winner here. All have advantages and
disadvantages. However, if you had a particular type of boating in
mind, the chart may help you make a choice. For blue water sailors, the
metal fin seems the way to go. Lots of righting ability. For inshore
sailors where a rare capsize won't mean death, perhaps the internal
water ballast, with its simple trailering abilities due to light
unballasted weight and very low draft, will be to your advantage. The V
tanked boat might be best for someone who cares a bit more about
ultimate stability. A combination of all of the above might be in order
for some folks.

Jewelbox

JEWELBOX, BEACHBOAT, 19' X 6', 800 POUNDS EMPTY

Someday I may get to put my full catalog on the net. For now I'll put
one design in each issue.

Here is a boat that is supposed to be self righting to a great degree
without ballast! Jewelbox has the combination proven by Bolger's
Birdwatcher of high sides with a thick bottom and low down crew
weight to act as ballast. Karl James told me his boat has righted
from having its windows totally submerged. Thanks to Tim Webber for
the scans. The Texas grapevine reports the original boat has
transferred hands as Karl has gone on to designing his own. No
surprize there since Karl had told me of adventures with 5 different
boats he had owned. His Jewelbox has been all over the country,
including the Mexican trip shown in the photo above, and a lake/canal
journey across the Florida peninsula. Here's the write up from my
catalog....

The Jewelbox prototype was built by Karl James of Winnsboro, Texas.
Here's Karl raising the sail. You can see how safe the operation is.
It's all done from inside the boat including stepping the mast.

In her first season this boat sailed in Mexico, the Apostle Isles of
Lake Superior, Montana's Fort Peck Reservoir, and Yosemite Lake. Karl
also brought it to our Midwest Messabout and I had a chance to sail
her. We had almost no wind that day and as we ghosted along I asked
him if the sail area was about right. He replied that it was right
for a Texas wind, hinting that everything was bigger and better in
Texas.

I watched Karl launch and rig Jewelbox solo in 15 minutes without
getting his feet wet. Stepping the mast is a 30 second operation and
it brings up one of the advantages of a lug rig. Jewelbox sets almost
160 square feet of sail on a 20 foot mast you can step solo without
strain. Not only is the lug mast shorter than most other types, it
can be lighter because mast deflection doesn't harm the set of the
sail as it does in other types. So rigging Jewelbox is about like
rigging a Sunfish. But this boat will sleep three adults inside the
self-righting Birdwatcher cabin with lots of dry storage under the
fore and aft decks. Karl trailers his with rudder and 6 hp motor in
place ready to go. There's a self draining well in the bow to hold
anchors,, muddy shoes, etc., with a step through bow transom.

I should mention that Karl made some beautiful segmented hard covers
for the walkway slot. But later he went to a simple snap on fabric
job. He greatly prefers the fabric job for camping. The hard covers
are used for trailering and storage.

Prototype News

Some of you may know that in addition to the one buck catalog which
now contains 20 "done" boats, I offer another catalog of 20 unbuilt
prototypes. The buck catalog has on its last page a list and brief
description of the boats currently in the Catalog of Prototypes. That
catalog also contains some articles that I wrote for Messing About In
Boats and Boatbuilder magazines. The Catalog of Prototypes costs $3.
The both together amount to 50 pages for $4, an offer you may have
seen in Woodenboat ads. (If you order a catalog from an internet page
you might state that in your letter so I can get an idea of how
effective this medium is.) Payment must be in US funds. The banks
here won't accept anything else. (I've got a little stash of foreign
currancy that I can admire but not spend.) I'm way too small for
credit cards.

Anyway..... Anytime a design from the Catalog of Prototypes starts
getting built I pull it and replace it with another prototype. So
that boat goes into limbo until the builder finishes and sends a test
report and a photo. There are three boats in that catagory right now.

Scram Pram, a 16' multichine Birdwatcher type, has been completed and
had its first sail near Savannah. I'm hoping for a good photo one of
these days. The builder said it's hard to get good photos of your own
boat and he's quite right about that. The surest way is to get a good
beach shot of it.

And Skat, a 12' cat boat daysailer, is being built near Phoenix.
Latest word from the West Coast Spy is that the boat has been glassed
on the outside and ready to flip and finish.

The Kansas Boat Psychologist keeps plugging away at his Fusebox. The
front porch is done. The hull has been flipped and the bottom
installation started. I'm leaving Fusebox in the Prototypes Catalog
for now because it has appeared on at least three web pages. It looks
as though the name "Fusebox" may give way soon to the name
"Harmonica" at the prototype builder's request.

Here on the AF4 home front, the boat is essentially done.

The painting, which is never really done on my boats, went OK. The
schedule was for two coats of primer and two coats of top paint. But
I found the plywood had such a good surface that one coat of primer
seemed sufficient. Besides, one coat over the whole hull pretty well
used up my gallon of primer. (Anybody know why oil based primer is
sold to prep wood for latex paint???) Below the wale, the hull is
painted dark Hunter green. Three coats needed to get sufficient
density here but I've still got half a gallon left. Above the wale
and interior are painted flat buff (Dairy is what they called the
color.) Only one coat seemed needed to totally cover the primer and I
still have 3/4 of a gallon left. I'm concerned that the flat color
will hold dirt too well. Our shores here are very muddy and keeping a
boat clean is very difficult. In that same line, I've varnished the
floors with three coats, two quarts being needed. (Couldn't find
gallon cans of oil varnish.) Varnished floors seem to hold up much
better and show dirt less than painted floors.

I mounted hardware and windows. Not much hardware compared to a
sailboat.

Started prepping my old trailer for the new boat according to the
stuff I wrote a few issues ago. Will make a point of weighing the
trailer before and after loading the boat, probably the first time
anyone in history has ever done such a thing.

This boat will have to live outside. So I'll need to make a custom
cover for it. I've done this before using polytarp, trimmed and sewn
to shape with a drawstring and tiedowns around the perimeter. I may
dart this one to make it form fitting. After that I'll need to dream
up a tarp cover for the cabin walkway. The bimini will be a summer
project.

Right now I'm 110 hours and $420 into the project. . With luck, AF4
will be launched in the first week of March. Actually we've had very
good weather lately for testing a new boat like this. But usually in
the Spring it gets so windy here that one must be quite careful.
Testing new boats on windy days is probably the biggest downfall for
the inexperienced. You can get in big trouble and lots of folks do.
You just HAVE to wait for a day with light winds.